This invention generally relates to analog circuits and systems and more particularly relates to high temperature coefficient communication circuits and systems.
Amplifiers are commonly employed within integrated circuits as components of a variety of analog signal processing circuits. However, variations in amplifier temperature may cause large variations in the transconductance (Gm) of field effect transistors (FETs) which are commonly used in analog processing circuits.
For example, the transconductance of an FET is typically inversely proportional to temperature, such that increases in device temperature decrease the transconductance of the device. Therefore, in Metal-Oxide-Semiconductor (MOS) design, it may be necessary to compensate for the temperature related effects on performance. Temperature compensation can be accomplished by altering the gate bias voltage of the transistor so that the gate bias voltage is modulated (up or down) when transconductance is altered by the effect of temperature. For example, when the transconductance is reduced under conditions of higher temperature, the gate bias voltage is increased to such a degree that the transconductance of the transistor is actually increased to reverse the effect of temperature.
In practice, if the transconductance of the device is kept relatively constant over temperature, the gain of the amplifier (determined by the product of the load impedance and the transconductance (gm)) remains relatively constant over temperature if the load has a relatively low temperature coefficient. In addition, the load of low frequency open loop circuits is typically a resistor, which, for many processes, may have a relatively low temperature coefficient. Therefore the performance of a low frequency system having a constant transconductance over temperature often remains relatively stable over temperature.
However, open loop loads at high frequency tend to be inductive to tune out the parasitic capacitance on the output node. The effective output impedance of the amplifier is therefore Q2R where Q is the quality factor of the inductor and R, the series resistance of a non-ideal inductor, which typically has a relatively high temperature coefficient. Therefore, the effective impedance of the inductive load varies with temperature as does the resulting transconductance of the device. This may result in a relatively large gain variation with varying temperature.